Great Exhibition of 1851. 163 



stract truths, however apparently remote from practice ; because 

 science only benefits industry by its overflowings, arising from the 

 very fulness of its measure. 



Every abstract truth, in its due time, adds to human resources 

 and enjoyments, and it is this text that I wish to inculcate from 

 examples derived from the Exhibition. One of the last generaliza- 

 tions of the great Berzelius, was that of allotropism, a name only 

 eleven years old, and fully explained by him only six years since ; 

 and yet this generalization, apparently at the time only of ab- 

 stract interest, entirely remote from practical application, produced 

 as fruit the three most original, and, I think, the most important, 

 practical discoveries of the Exhibition. 



Having thus introduced the subject of his lecture, Dr Playfair 

 proceeded to offer certain examples of allotropism. It had long 

 been known that bodies crystallised in two or more incompatible 

 forms. Thus, carbonate of lime as arragonite crystallises in prisms ; 

 whereas as calcareous spar it crystallises in rhombs. Sulphur also 

 crystallises in two incompatible forms ; so does the garnet. This 

 is termed dimorphism. When two such forms exist they are 

 found to be maintained in unequal stability ; it appears, in fact, as 

 if one form was normal and the other forced or strained. Thus a 

 prism of arragonite is subject to change into rhombs of calc spar ; 

 and sulphur crystallised by heat in oblique rhombic prisms passes 

 in a few days into a mass of rhombic octohedrons. Not only may 

 the chemical and physical characteristics of such dimorphous bodies 

 differ, but their colour and their specific gravity. Thus, the sul- 

 phuret of iron (Fe S 2 ), when crystallised in cubes, is persistent in 

 the air ; but when occurring in a rhombic form, readily passes into 

 copperas or sulphate of iron. 



Applying the preceding remarks to non-crystallised bodies, it was 

 equally found that many were susceptible of allotropic modification. 

 Thus cinnabar and vermilion were of precisely similar chemical 

 composition with the black sulphuret of mercury. Again, the 

 sesquisulphuret of antimony might be black or orange. Iodide of 

 mercury is commonly red ; when heated, however, it passes into 

 a yellow powder, which by simple pressure and rubbing with a hard 

 body becomes red again. Sugar is a remarkable instance of a 

 solid capable of assuming two allotropic states ; as sugar candy it 

 is crystallised, as barley sugar it is amorphous ; yet the composi- 

 tion of sugar in either case is the same. Nor are liquids exempt 

 from the strange state of allotropism, — sometimes indeed mani- 

 festing a condition even beyond allotropism (isomerism), and not 

 allowing us to reconvert them to their primitive state. Thus the 

 chemical composition of oil of turpentine, of rosemary, of lemons, 

 of copaiba, are identical, yet no one of these bodies has hitherto 

 been turned into the other. The steroptane of otto of roses is 

 identical in composition with coal gas, yet chemists are unable to 

 change one into the other. The term isomerism has been cora- 

 ls 



